The present invention relates to turbomachinery and axial flow compressors. More particularly, the present invention pertains to a shroud leakage cover, which can be applied to the inner shroud region of stator vanes in a compressor of a gas turbine engine. The shroud leakage cover protects against direct impingement of the leakage air onto a leading edge of the stator vanes and degraded compressor performance resulting therefrom.
Gas turbine engines have been utilized to power a wide variety of mechanical drives for vehicles and electrical power production. The operation of a gas turbine engine can be summarized in a three-step process in which air is compressed in a rotating compressor, heated in a combustion chamber, and expanded through a turbine. The power output of the turbine is utilized to drive the compressor and any mechanical load connected to the drive. Axial-flow compressors may comprise a plurality of annular disk members carrying airfoils at the peripheries thereof. Some of the disk members are attached to an inner rotor and are therefore rotating (rotor) blade assemblies while other disk members depend from an outer casing and are therefore stationary (stator) blade or vane assemblies. The airfoils or blades act upon the fluid (air) entering the inlet of the compressor and raise its temperature and pressure preparatory to directing the air to a continuous flow combustion system. The stator vanes redirect and diffuse air exiting a rotating blade assembly into an optimal direction for a following rotating blade assembly. The air entering the inlet of the compressor is at a lower total pressure than the air at the discharge end of the compressor, the difference in total pressure being known as the compressor pressure ratio. Internally, a static pressure rise occurs across the stator vanes from diffusion and velocity reduction.
For a number of reasons having primarily to do with the design parameters of the cycle utilized in a particular engine, it is undesirable for the higher static pressure and higher static temperature air at the discharge side of a stator vane assembly to find its way back into the primary air flow at the inlet side of the stator vane assembly. This air, which returns to the relatively low static pressure area at the vane assembly inlet, is called leakage air and results in reduced engine efficiency. Leakage of air within the compressor thus detracts not only from the efficiency of the compressor itself, but also the overall efficiency of gas turbine engine operation.
Labyrinth seals connected radially inward from the stator vane assemblies of the compressor stage and sealing against the inner rotor have long been utilized as a means to prevent leakage flow about the primary working fluid path around the stator vane assemblies. Notwithstanding the use of labyrinth seals, some leakage does occur, and this leakage air will travel, for example, from the high static pressure downstream side of a stator vane assembly to the lower static pressure at the upstream side of the stator vane assembly via a path which exists between the radial inward end of the stator vane assembly and the labyrinth seals connected to the rotor. After traveling to the upstream side of the stator vane assembly, the leakage air travels in a radially outward manner in the cavity existing between the stator vane assembly and adjacent rotor assembly. This radial path taken by the leakage air has a tendency to reduce the velocity and axial direction of air traversing the working fluid flow path of the compressor and tends to increase the amount of bleed air which further contributes to engine inefficiency.
Efforts have been made (Walker et al. in U.S. Pat. No. 5,211,533) for diverting leakage air back into the flow path of a turbine engine. A stator vane assembly may be connected to a shroud assembly at the radially inner end of the stator vane assembly. The shroud assembly is provided with a scoop, which is placed in the path of leakage air traversing in a forward direction from the high-pressure static side of the stator vane to the low static pressure side of the stator vane. The leakage path is located between the stator vane assembly and a rotating member. The scoop intercepts the leakage air and re-directs the leakage air into an airflow path of the compressor with an aftward component of velocity.
However, the leakage flow that is coming into the flowpath in the radial direction has a strong negative impact on the axial momentum of the fluid in the vicinity of the injection. The reduction in axial momentum increases the loading on the leading edge of the airfoil, which can lead to separated flow and compressor surge. It would be desirable to eliminate this adverse impact on the leading edge of the stator vane while maintaining the axial component of velocity imparted to the leakage air returning to the compression flow.